1. Field of the Invention
The present invention refers to roofing granules and roofing products.
2. Brief Description of the Prior Art
Asphalt shingles are conventionally used in the United States and Canada as roofing and siding materials.
Asphalt shingles can be classified into two types of shingles according to the nature of the reinforcement. “Organic” shingles contain cellulose or wood fiber as a thick fiber felt. “Glass fiber” shingles contain a nonwoven mat of glass fibers held together by a binder that is insoluble in water.
In the manufacture of organic shingles, a continuous web of organic fiber felt is fed from a supply roll to an accumulating device made up of several rollers, and then immersed in a first liquid asphalt bath having a temperature of about 250 degrees Celsius. After leaving the first liquid asphalt bath, the felt passes through a second accumulating device so that the felt can absorb excess asphalt and cool slightly. The so-impregnated felt is then coated with molten asphalt on each of its two faces, which ultimately become the upper and lower faces of the web respectively. Roofing granules are distributed on the upper face, and an anti-adhesive agent, for example, talc, is applied to the lower face. The resulting web passes between the rollers of a cold calender so as to partially embed the roofing granules in the hot asphalt layer on the upper face of the web, and the subsequently cooled product is collected in the form of rolls or of sheets cut to the desired dimensions.
Except for the first stage of impregnation, which is omitted, the manufacture of the glass fiber shingles is carried out in the same way.
In the shingle, the asphalt functions principally to make material impervious to water. It is also used to support the granules and to give strength to the material. The highly ductile character of the asphalt-impregnated felt makes it possible to obtain a flexible product. In general, the longevity of the shingle increases with the quantity of asphalt employed.
The roofing granules, in general formed from mineral materials, serve to provide the shingle with durability. They protect the asphalt from the effects of solar radiation (in particular, from the degradative effects of ultraviolet rays) and of the environment (wind, precipitation, pollution, and the like), and contribute to better reflection of incident radiation. The granules, moreover, are typically colored, naturally or artificially by the application of pigments, to meet the aesthetic requirements of the user.
Roofing granules are typically manufactured by using suitable mineral particles coated with coating compositions formed from mixtures of inorganic metal-silicate binders, extender, and metal oxide colorants. Examples of making such granules are disclosed, for example, in U.S. Pat. No. 2,981,636. However, such coating processes typically require curing at elevated temperatures, such as above 300 degrees Celsius, or the use of extensive acid wash to render the coating durable for roofing applications, and provide porous coatings. As a result, many colorants or functional additives, which are not resistant to high temperatures or to the corrosive acid-wash process, cannot be used. This limits the palette of colors which can be used in manufacturing roofing granules. Furthermore, the type of coating formed in the conventional process is known to be prone to the so-called “staining” problems as this type of coating tends to absorb oils from the asphalt substrate, apparently due to the porosity of the coating.
International Patent Publication WO 2006/106263 A2 discloses biocidal granules consisting of a mineral core coated with at least one porous inorganic coat containing at least one organic compound adapted to limit or to prevent microorganism growth, in particular algae, as well as a method for sol-gel preparation of such biocidal granules and the use of said biocidal granules in building materials, in particular shingles and façade coatings.
U.S. Pat. No. 5,723,516 discloses inorganic particles coated with a composition including an organometallic polymeric binder and a temperature-sensitive colorant. The binder comprises the reaction product of water with at least one component of a binder precursor. The binder precursor composition can comprise an inert organic polymer and a hydrolyzable liquid organometallic solvent such as TEOS; a water-reactive organic polymer dissolved in an inert organic solvent; or a water-reactive organic polymer dissolved in a hydrolyzable liquid organic solvent.
U.S. Pat. No. 6,786,965 discloses an organic pigment dispersion for coloring building material such as concrete, including alkali metal silicate particles, organic pigment and a dispersant which is milled to produce a pigment particle size of from 100 to 300 nanometers.
Further roofing granules produced by the methods disclosed, for example, in U.S. Pat. No. 2,981,636 are found to have a porous surface and the luster of the surface appearance is always dull and non-shiny. Since the shiny, glassy appearance of the surface can become very appealing in some black, deep dark colors, people have been searching for alternative surfacing media in order to provide the needed aesthetic value and functionality for roofing shingles.
For example, certain amorphous slag materials, such as copper slag or coal slag, have been crushed into suitable size and used as roofing granules known as “Black Beauty” roofing granules, available from Harsco Technologies Corp., Fairless Hills, Pa. These slag materials have a composition including iron, aluminum and calcium silicates and provide a desirable glassy, luster appearance in black color that provides high aesthetic value.
U.S. Pat. Nos. 6,746,636 and 6,258,456 disclose forming slag-like roofing granules by firing glassy ceramics with certain compositions, similar to the formation of slag materials in the typical smoldering process. The materials are then crushed into suitable sizes for roofing applications.
The supply of Black Beauty granules has been diminishing due to the reduction of the waste stream from which they are derived. In addition, the slag materials often contain undesirable amounts of heavy metals or iron that either cause environmental concerns or discoloration due to so-called “rusting.” Further, no color available other than black provides roofing granules with a glassy, shiny lustrous appearance, which can be a desirable aesthetic for roofing.
Hence, it would be very advantageous if roofing granules could be colored using non-slag base materials to provide a surface with a glassy, shiny appearance with the desirable luster.
In addition, it would be advantageous to have an inorganic coating system that can be cured at relatively low temperatures. It is further advantageous to have an inorganic coating that provides better staining resistance without the need of additional surface treatment.